Shallacatop was one of the planets that the Doctor mentioned that were stolen by the Daleks in The Stolen Earth/ Journey's End though no details were given other than its name (that what my research says anyway). So all other details are mine, apologies if I have missed any details about the planet given by the show.
"Welcome to Shallacatop!" exclaimed the Doctor as she flung open the doors of the TARDIS. A blast of hot air wafted her coat and mussed her hair. She stepped out, closely followed by the Fam, Yaz and Ryan surveying the landscape with awe as Graham shielded his eyes with a hand.
"It's amazing!" said Yaz, looking up at the nearby mountains, their snow capped peaks sparkling emerald green under the golden sky.
"This is one of a pair of planets that orbit around each other as they travel around their star," explained the Doctor, hands and fingers waving in the air as she talked. "A binary planetary system. The Daleks stole it, along with Earth and 26 other planets. But I was able to put them back into their proper positions again."
"Stole the Earth?" questioned Graham. "Is that the time there was an earthquake and it suddenly turned into night in the middle of the morning? I'd just started my shift on the bus and then suddenly I could see stars. In the middle of the mornin'!" The Doctor nodded.
"I remember the earthquakes," said Yaz, "and hiding under the desks at school. I must have been about seven, eight."
"I'd forgotten about that," Ryan added. "It were proper scary."
"How can a planet be stolen, Doc?" asked Graham. The Doctor shoved her hands in her pockets and rocked back onto her heels.
"Long story short is that the Daleks wanted to destroy the universe and placed all the planets around the time rift in the Medusa Cascade to amplify the reality bomb they were going to use. But I stopped them, with the help of some friends. Then I used the TARDIS to tow the Earth back into its proper orbit. The old girl can do more than travel through time, though she wasn't happy with me for a while, she didn't like being used as a work horse. But giving her a bit of an overhaul got her out of her mood and-"
"ON YOUR FEET, PRISONER!"
The Doctor gasped in surprise, head shooting up from its slumped position on her chest. She looked around in confusion as she surveyed the grey stone walls, heart sinking as she remembered the cell she was contained in, the daydream shattered and fading. She had never taken anybody to Shallacatop, let alone the Fam.
"ON YOUR FEET, PRISONER!" the disembodied voice repeated.
She clambered to her feet, limbs aching from disuse, the feeling of happiness and togetherness sinking into depression, loneliness and despair. Who knew when she would see the Fam again? Would she ever get out of this cell?
She didn't know.